Retrieving the time-critical target

Effective combat search and rescue requires more than mere lip service

Combat search and rescue (CSAR) has long been the orphan of military operations. As with any form of insurance, nobody wants to pay the premiums until a mishap occurs. In many conflicts over the past 30 years, the sight of downed aircrew being paraded on television by their captors - sometimes accompanied by a threat to use them as 'human shields' - has caused an outpouring of public outrage. This factor may well now outweigh purely military reasons for attempting to retrieve personnel from behind enemy lines. Present CSAR operations suffer from deficiencies in terms of the time taken to reach survivors and limitations of the rescue platforms themselves. The former is a function of the limited near-real-time connectivity among sensors, weapons, survivors and recovery forces, together with shortfalls in the timely co-ordination and execution of CSAR operations. Platform limitations include performance (in terms of range, speed and altitude), vulnerability (limited on-board situational awareness, inadequate defensive aids, and aircraft survivability), and the deployment and employment footprint (which often depends on strategic airlift and air-to-air refueling).

The US Air Force (USAF) Aerospace Command and Control, Intelligence, Surveillance and Reconnaissance Center has developed a roadmap that it hopes will, by 2008-10, allow the service to engage time-critical targets (TCTs) within 30min (from detection to weapon impact). This will affect all aspects of CSAR: the survivor becomes a TCT in his or her own right; and with the availability of support aircraft performing combat air patrol, strike and suppression of enemy air defenses missions at short notice increases the chance of success.

This latter point becomes increasingly important as defending forces are better equipped and organized. A study conducted in 1996 indicated that the combined total of search parties, anti-aircraft artillery units and man-portable air-defense systems that would probably be deployed by Iraq in the area of a downed aircraft at night rises from virtually zero after 30min to about 13 within an hour, and approaches 25 within 2.5h.

Despite the limited budgets and lack of high-ranking enthusiasm generally associated with CSAR, however, several air arms are introducing new equipment in this field or are including a CSAR capability in their requirements for multirole helicopters. The major contenders in this field are the AS532 AL variant of Eurocopter's Cougar Mk2, the NH Industries NH90, the EH Industries EH101, versions of the Sikorsky H-60 and its S-92 derivative, the Bell Boeing V-22 Osprey tilt-rotor, and the Russian Mi-8/Mi-17 family.

The first four of these are competing in the Nordic Standard Helicopter Programme (NSHP), for which bids were submitted in April. The NSHP involves the supply of 73 helicopters to equip the armed forces of Denmark, Finland, Norway and Sweden, specified roles for which include both CSAR and more general SAR operations. Portugal has a requirement for 12 new SAR helicopters; Germany plans to operate some of its NH90s in the CSAR role; and the Italian Air Force is forming a special operations unit for CSAR and other roles, using HH-3Fs.

In the US, Air Combat Command - which assumed the USAF's responsibility for CSAR in 1996 - has launched an upgrade program for its fleet of HH-60G Pave Hawks and is seeking a longer-term replacement. Contenders being evaluated under an analysis of alternatives include the EH101, S-92 and V-22.

Eurocopter has developed the AS532 AL in collaboration with the French Air Force, which took delivery of its first such aircraft last year. A further three are due to follow by 2003, with an eventual requirement for 14. The helicopter can be refueled in flight, permitting 8h missions involving a 740km transit at 130km/h, followed by 30min in the rescue zone and then a return to base. Eurocopter is also delivering 12 Cougar Mk2s for CSAR to Saudi Arabia, and Greece recently ordered four. The latter country has also introduced four Eurocopter AS332 C1 Super Pumas, which it operates on maritime surveillance and SAR missions. Their role equipment, which is similar to that specified as standard for the Cougar Mk2, includes a Telephonics RDR 1500B radar, a Thomson-CSF Chlio forward-looking infrared set (FLIR) and a Spectrolab searchlight.

The Turkish Air Force is taking delivery of 20 AS532 ALs for CSAR as part of the FFr4.45 billion (US$33 million) Phenix 2 contract, awarded to Eurocopter in 1997, that also includes 10 AS532 UL utility helicopters for Turkish Army Aviation. The EUROTAI consortium set up by Eurocopter and TUSAS Aerospace Industries is building 28 of the helicopters, deliveries of which are due to be completed in February 2003.

The first of 15 EH Industries Cormorant SAR helicopters for the Canadian Forces made its maiden flight in May. Deliveries are due to begin next February and be completed within two years. Role equipment includes a Telephonics RDR 1400 radar, a Spectrolab SX-16 searchlight and a Breeze-Eastern twin rescue hoist system. The helicopter has a patient treatment area with three stretchers on the port side, and the main cabin can accommodate 12 stretchers.

Earlier this year, an EH101 conducted a simulated SAR mission covering 945nm (1,750km) and lasting more than 8h in support of Portugal's requirement to conduct long-range rescue of the crew from a P-3 maritime-patrol aircraft. The aircraft took off at a weight of 15,600kg, with 5,500kg of fuel in the internal and auxiliary long- range tanks. At a height of 2,000ft (610m) it shut down one of its three engines, cruising at 120kt on the remaining two. At a distance of 400nm (750km) it then performed a 30min hover out of ground effect, representing a typical rescue mission, before returning.

The US armed forces are implementing programs to correct some of the deficiencies highlighted by the Joint Combat Search and Rescue (JCSAR) Joint Test and Evaluation (JT&E) program. This three-year effort, completed in December 1998, examined the entire JCSAR end- to-end process from the initial report of a downed aircraft to the recovery of isolated personnel. It also included testing the effectiveness of individual functions - location and identification; surface-based command, control, communications, computers and intelligence (SBC[4]I); and mission execution - by means of data from a combination of command post exercises, field training exercises and virtual simulations.

The resulting report concluded that the mean survivor location error was 6.5nm and the mean C[4]I processing time was 7.3h. This did not include the time taken to recover a survivor. Of 45 baseline missions examined, 22 were successful in recovering a survivor without causing any secondary losses. Other trials were either only partially successful or failures.

The implementation of better training (for Joint Search and Rescue Center staff, airborne mission commanders and the aircrew acting as 'survivors') during the JT&E, together with new equipment, led to improvements. The mean survivor location error dropped to 0.5nm (although still short of the 100m desired), and the C[4]I processing time almost halved to 3.8h. The mission success rate of less than 50% remained about the same, however, and working groups organized by the JT&E expressed a desire to reduce end-to-end JCSAR mission duration to 2h or less. The main ways of achieving this were judged to be further improvements in training, and the introduction of the Combat Survivor Evader Locator (CSEL) system.

CSEL is now due to begin full-rate production in the third quarter of Fiscal Year 2002 (FY02). The US armed forces plan to buy approximately 50,000 units of the AN/PRQ-7 hand-held radio (HHR), which will replace the current AN/PRC-90 and AN/PRC-112/112B survival radios. In addition to the HHRs, CSEL includes unattended UHF base stations (UBSs) for over-the-horizon (OTH) communication and tracking, and ground support segment software for the Joint Search and Rescue Centers (JSRCs). The HHR, which incorporates a Global Positioning System (GPS) receiver, provides UHF/VHF line-of- sight (LOS) voice beacon facilities and three OTH data modes: two- way secure UHF satellite communications (satcoms), a one-way secure link using national assets, and one-way non-secure transmission via the Search and Rescue Satellite Assisted Tracking (SARSAT) system for polar coverage. The precise GPS position is included in OTH data communications.

The OTH segment includes four UBSs, which control two-way worldwide UHF satcoms with multiple HHRs. They also interface with national assets, SARSAT and multiple JSRCs using the SIPRNET secure network. The ground segment consists of the segmented software application, which receives and transmits messages from/to the HHR through the UBS.

The initial operational assessment of CSEL, conducted in 1998, concluded that the system needed to be made simpler and more reliable. As a result, the program was restructured to include a second operational assessment (OA) in the fourth quarter of FY00. Boeing built 100 radios incorporating changes intended to remedy these deficiencies, which it demonstrated in September 1999, together with a further 90 for participation in the second OA. Modifications to the HHR include a new screen and keypad, the addition of status and help displays, a simplified menu structure and new software. Boeing has also redesigned the VHF/UHF, GPS and controller modules to improve their reliability and remedy other shortfalls.

Initial operational test and evaluation (IOT&E) is now scheduled for the first quarter of FY02. Aspects to be assessed during IOT&E include unattended operation of the UBS, and changes resulting from the introduction of additional facilities that should further improve the system's message-delivery performance.

USAF missions

The USAF's Air Combat Command (ACC) and Air Force Special Operations Command (AFSOC) conduct land-based CSAR missions. The latter nominally performs such a role on behalf of Special Operations Forces (SOF) only, but also provides assistance "in support of conventional forces on a case-by-case basis, [so as] not to interfere with the readiness or operations of core SOF missions". For example, AFSOC led the efforts to rescue the pilots of both USAF aircraft (an F-117 and an F-16) shot down during last year's Operation 'Allied Force' in Yugoslavia.

ACC operates 105 HH-60G Pave Hawks in the CSAR role. Modifications include a hover autopilot, an air-refueling probe, an external hoist that can recover survivors (which may be strapped to a Stokes litter) from a hover height of up to 200ft (61m) and can be fitted with a forest-canopy penetrator, and an IR strobe light. AFSOC also operates the Pave Hawk in its MH-60G variant, which incorporates substantial additional facilities to support its primary wartime mission of inserting, extracting and resupplying special forces (including operations at night and in bad weather).

Sikorsky has modified an HH-60G to Block 152 standard by incorporating the Upgraded Communication, Navigation/ Integrated Electronic Warfare (UCN/IEW) suite. The prototype is due to complete six months of trials by the service, involving developmental and operational test and evaluation, at the end of the year. The USAF had planned to award Sikorsky a contract to implement the upgrade aboard a further 48 aircraft by 2007, but this is now unlikely to proceed.

AFSOC plans to replace its MH-53J Pave Low helicopters with 50 CV-22 Ospreys, roles for which will include CSAR. The CV-22 can carry 1,800kg more fuel than the MV-22 variant, increasing its flight time by 1.5h. Bell Boeing has converted two MV-22s from the engineering and manufacturing development program to act as test aircraft. The first of these has been remanufactured to CV-22 standard, including the installation of dedicated equipment such as a multimode radar, integrated electronic-warfare (EW) suite and auxiliary fuel tanks. The second aircraft has undergone a less extensive modification. These were delivered during September to Edwards Air Force Base in California, where they are being evaluated by the CV-22 Integrated Test Team. This will culminate in operational test and evaluation, scheduled for August 2002. Procurement of production aircraft is due to begin next year, with deliveries starting in 2003.

Initial operational capability (IOC) with six CV-22s was scheduled to have been achieved in September 2004, with all 50 aircraft being in service by 2009, although the program has recently been stretched and restructured for a variety of reasons. These include the US Army's decision to defer procurement of the Suite of Integrated Radio Frequency Countermeasures (SIRFC) EW system for its helicopters until at least 2006, and limits placed on development funding. The CV-22 may now not achieve IOC until 2007-08, and some items that were formerly planned as base line equipment - such as the DIRCM self-defense system - will be implemented under a preplanned product-improvement program leading to the Block 10 variant.

Bell Boeing contends that the aircraft's design eliminates CSAR platform limitations by virtue of its ballistic tolerance, EW suite, low signatures, surveillance sensors, protection against directed- energy weapons, situational awareness, NBC (nuclear, biological and chemical) protection, and self-defense. Measures contributing to the first of these include the use of composite materials, fire protection, systems redundancy and armor.

The EW fit includes the AN/ALQ-211 SIRFC and AN/AAQ-24 Directed Infrared Countermeasures (DIRCM) set, augmented by missile/laser warners and countermeasures dispensers. SIRFC additionally acts as the controller for the EW system's own dedicated MIL-STD-1553B databus. It can store and manage data for up to 128 survivors, using inputs (including those from CSEL) that the MATT receives via satellite communications links. SIRFC then feeds data back to the Advanced Mission Computer for control and display.

New surveillance sensors include the AN/AAQ-27 FLIR and AN/APQ-186 Ku-band Multi Mode Radar (MMR). The latter permits terrain- following/terrain- avoidance (TF/TA) operation down to 100ft (30.5m). Self-defense includes the nose-mounted Turreted Gun System (TGS) and a ramp gun, with options for a laser rangefinder/ designator and automated target hand-off system. In August, Bell Boeing selected General Dynamics Armament Systems to provide the TGS that will arm all Ospreys (including MV-22s). The TGS incorporates a GAU-19/A three-barrel 0.50-caliber Gatling gun with 750 rounds, and a linkless ammunition handling system. The weapon can be reloaded in flight from the cargo bay. The gun, turret and feed - which are all electrically powered - are integrated with the stabilized FLIR and the gunner's helmet-mounted sighting system via the MIL-STD-1553B databus. The TGS can cover a forward arc of 74 on each side of the aircraft's nose, and depress to -50.

Candidate operating procedures for the CV-22 include 'strip alert', 'safe loiter' and 'forward loiter'. The first of these involves forward deployment to an alert base approximately 50n miles from the hostile border, with the aircraft being ready for launch at 15min notice. Safe loiter involves the aircraft orbiting near the border, with forward loiter extending this to an orbit (in as low-threat an area as possible) over enemy territory. In the case of forward loiter, three CV-22s flying orbits at an altitude of 18,000ft (5,550m) - above the ceiling of low-level air-defense threats - could provide CSAR coverage of the whole of southern Iraq within the 30min limit foreseen for the engagement of TCTs. Once alerted by CSEL, the tilt-rotor would spend 3min descending at 300kt to 200ft (61m), followed by 25min at 230kt using the TF/TA radar, then slow to 40kt in the vicinity of a survivor. It would remain in the area for up to 15min, in the hover or at low speed (up to 40kt), before repeating the process in reverse and recovering to base.

Simulations of such concepts of operations conducted using the Advanced Tactical Combat Model indicate that the probability of rescue is more than 0.6 with forward loiter, decreasing to 0.45 with safe loiter and 0.3 with strip alert. The corresponding figures for rescue time are just under 30min, approximately 50min and more than 80min respectively, with CV-22 attrition losses being in about these same ratios. A further possibility involves 'precautionary' CSAR operations to provide coverage within a limited geographical area in advance of a major offensive when significant combat losses are expected. This may involve aircraft orbiting, or ready for rapid launch in strip alert.

DERA analysis

In 1997, the UK Defence Evaluation and Research Agency commissioned ANSER, a US public-interest research organization, to conduct a mission effectiveness analysis of the H-60 and V-22 in the CSAR mission. The authors used a computer model to apply projected realistic loss rates to actual sorties flown by Coalition aircraft during the 1990-91 Gulf War (in which the losses incurred were regarded as being unrealistically low). Staff members from the US Joint Combat Rescue Agency, ACC and AFSOC contributed expertise, access to mission-planning software, and comments.

The study involved notional rather than specific models of the H-60 and V-22, in configurations optimized for CSAR (including the installation of terrain-following/terrain-avoidance radar and defensive systems in each case). The V-22 was also constrained to operate under concepts developed for helicopter operations, such as launch from strip alert.

The conclusions were:

- The V-22 is significantly more effective than the H-60 in conducting CSAR under the conditions studied. The Osprey achieved approximately three times the number of saves per downed rescue crewmember, with this ratio increasing to between 4:1 and 7:1 when taking into account additional factors such as the tilt-rotor's unique capabilities.

- The more demanding the rescue requirement, the greater the advantage of the V-22. Longer distances and higher threat environments further increase this differential.

- Use of the V-22 eliminates dependence on aerial refueling. In the study's base case, this saved 107 tanking operations (36 of which would have been behind enemy lines).

- The V-22 was far more efficient in terms of resources, requiring far fewer airlifter sorties for deployment and significantly reduced numbers of personnel.

A mission-needs statement issued in 1997 identified deficiencies in the USAF's current CSAR force in terms of service life, vulnerability, penetration in adverse weather, combat radius, situational awareness, payload space and reaction time. ACC is, therefore, conducting an analysis of alternatives (AoA), the final results of which are due to be published in March 2001, to determine the best method of sustaining its CSAR capability. The AoA is based on the need to conduct CSAR operations during a major theater war lasting 30 days. Sortie parameters are determined from historical data, which show that the number of recoveries required would typically be six each on the first two days, dropping to an average of three a day after the first week. Only 6% of rescue sites are at a distance of more than 200n miles, and the number of personnel at each site - based on the crew complement of USAF aircraft - is either one or two on 88% of occasions.

The AoA is considering alternative platforms in four categories: variants of the H-60, other currently available equipment, new US developments, and overseas contenders. The first of these includes further modernization of the HH-60G, building new HH-60Gs and other variants of the family: the H-60X, UH-60Q, MH-60K, the US Coast Guard's HH-60J and the US Navy's CH-60. The second category encompasses the MH-53J/M, CH-53E, MH-47E and UH-1Y. New US platforms under consideration include the CV-22, S-92, BA-609 and DP-2. Overseas competitors include the AB319, Mi-17, Cougar Mk2, EH101 and NH90. ACC requires a total of 120 to 130 aircraft, which could involve more than one type (a helicopter, for example, augmented by a small number of tilt-rotors for deep missions). If a new type is deemed necessary, IOC - with approximately six aircraft - would be achieved at the end of FY07 and full operational capability 10 years later.

EH Industries demonstrated the EH101 to ACC during May in support of the AoA. Advantages claimed for the helicopter include its large cabin, long endurance without a requirement for external fuel tanks (which can degrade downward visibility and hamper gun firing), low vibration levels, high performance in icing conditions, low maintenance and early delivery. The cabin provides the space needed to accommodate rescue personnel and survivors, together with the ancillary equipment that is increasingly being specified. The latter includes a telemedicine package, incorporating real-time video links, that draws on technology developed for the commercial emergency medical services market. Other payloads can include all- terrain vehicles and rigid inflatable boats, together with the search parties needed to operate them.

The large size of the side door, which measures 1.5mx1.5m, allows a crew member to stand while assisting the recovery of a survivor on a litter. In smaller helicopters, personnel frequently suffer debilitating injuries to their knees and lower backs as they maneuver in a confined space.

The active vibration-control measures incorporated in the EH101 provide a smooth ride for the avionics as well as for the crew, which contributes to a figure of 3.5 maintenance man-hours per flight hour (compared with approximately 40 for the H-53). The EH101 fleet has accumulated more than 15,000h of flight time, and EH Industries says that it - in conjunction with a US partner - could deliver at least three-quarters of the number required by ACC by the nominal date of FY07 foreseen for IOC. The aircraft's gross weight could grow by 1,800-2,700kg over the next few years, allowing it to accommodate additional facilities. EH101 has already examined the addition of a TF/TA radar, and the Merlin Mk3s being introduced by the UK Royal Air Force already carry a comprehensive defensive-aids system that includes the same DIRCM as that planned for AFSOC's CV- 22s.

The demanding requirements associated with search and rescue have led to the development of specialized equipment for this role. Canada, with its vast territory and long coastline (much of which is subjected to inhospitable Arctic conditions), has pioneered many such innovations. The air-dropped Self-Locating Datum Marker Buoy (SLDMB), developed by Seimac, contains a GPS beacon and an ARGOS satellite transmitter that provides data in real time to a Rescue Co-ordination Centre (RCC). The buoy drifts at the same speed and in the same direction as a liferaft or person in the water, thereby reducing the area that has to be searched for survivors. During its first operational use by the Canadian Forces, in October 1997, the SLDMB contributed to the successful rescue of a crewmember who had spent 19h in the water after his ship sank about 450 miles off Newfoundland. The RCC in Halifax incorporated the SLDMB inputs into its CANSARP (Canadian Search & Rescue Plan) software package, which indicated that - contrary to expectations - the two buoys were drifting against wind and seas, as a result of eddies in the Gulf Stream. The rescue ships adjusted their search area accordingly, and picked up the survivor.

Canada's Defence Research Establishment Valcartier (DREV) has developed a family of electro-optical sensor demonstrators with potential SAR applications. These include the Airborne Laser-Based Enhanced Detection and Observation System (ALBEDOS), which was designed to improve the effectiveness of SAR missions in poor visibility. The initial objective was to detect a person at a distance of 1.5km from an altitude of 1,640ft, identify a ship at the same range, and detect a co-operating target at 5km. WESCAM and the National Institute of Optics built a prototype that underwent flight trials, organized in collaboration with the National Research Council, aboard a Bell 412 in 1995.

ALBEDOS employs active range-gated imagery to overcome many of the limitations suffered by current surveillance equipment. SAR operations frequently must be carried out at night, often during very unfavorable weather conditions when even the most sensitive low-light-level television system is blinded by insufficient light, poor atmospheric transmission or heavy backscatter from fog or snow, and when infrared imaging systems are limited by low temperature contrast. ALBEDOS combines a pulsed near-infrared light source with a gated, intensified charge-coupled-device (CCD) camera sensor to permit detection and identification day and night, in virtually all weather conditions.

DREV is now developing enhanced versions of ALBEDOS and integrating them with complementary systems for a variety of applications. Canada's National Secretariat for Search and Rescue is funding development of the Enhanced Low-light-level Visible and Infrared Surveillance System (ELVISS), flight demonstrations of which, aboard a Bell 412, are planned for next summer.

ELVISS involves sensors mounted on two WESCAM stabilized platforms that are boresighted and aligned with each other. One, using a 61cm gimbal, carries an enhanced version of the ALBEDOS package. This incorporates a new laser illuminator, developed by INO (formerly the National Institute of Optics, which now functions as a commercial company), with twice the power of the original design and operating at a longer wavelength to permit more covert use.

The enhanced ALBEDOS also includes a video tracker, laser rangefinder (LRF), georectifying system, a new zoom lens and an improved electronic controller. The LRF automatically positions the laser illuminator's range gate over the area of interest, and feeds the target's latitude and longitude to the georectifying system with an accuracy of 10m. A third-generation implementation could include an eyesafe LRF. The other stabilized platform, on a 41cm gimbal, carries a FLIR with a x8 zoom lens, providing fields of view of 20x13 and 5x3.3. The ELVISS man/machine interface consists of a moving-map display in the cockpit.

Canada's New Search and Rescue Initiative program is also sponsoring the Infrared Eye project, which involves DREV, ProConcept and INO. The sensor has two fields of view (FOVs) that can be viewed simultaneously. The narrow, high-resolution FOV can be positioned anywhere within the wider field to inspect objects in it more closely. Both fields are covered by staring two-dimensional arrays, eliminating the complexity and signal loss of scanning systems. A fast opto-mechanical moving-mirror arrangement is used to position the narrow field.

Separating the two functions of detection and identification in this way allows each system to be optimized for its role. The initial prototype uses 256x256-element cooled indium antimonide detectors for both FOVs; the 6 narrow field, which is microscanned for higher resolution, is steered in the 60 wide field manually, with a joystick. The display system uses micro-mirror technology to display the fused images on a screen.

A second prototype employs quantum-well detectors developed in co- operation with the Institute of Microstructural Sciences. These provide many of the characteristics of multi-element array units, but their monolithic construction eliminates the between-pixel losses of arrays and the need for microscanning to attain high resolution without aliasing effects. The display unit uses a cathode-ray tube of very high resolution, with an eye-tracking module to steer the narrow FOV automatically along the operator's line of sight. The wide and narrow fields of view are 40 and 10 respectively.

Israeli company Tadiran Spectralink has supplied its Airborne Search And Rescue System (ASARS) to several customers, including France, Italy and Thailand. Recent orders include one from Sikorsky for seven systems to equip Turkish Army UH-60s to be employed on CSAR missions. ASARS consists of the ARS-700 airborne installation, with a range of more than 200km over a full 360 and an accuracy of a few meters in the final stages of approach to a survivor, and a PRC-434A hand-held transponder/radio carried by the pilot. Tadiran Spectralink claims that the use of an advanced and very short interrogation/response prototol eliminates the risk of detection suffered by alternative direction-finding approaches that require continuous beacon transmissions. The ASARS-G variant employs the upgraded PRC-434G, incorporating a remotely activated GPS receiver, operating in conjunction with an aircraft-mounted ARS-700G. It can also include the ARS-434R, a modified PRC-434A for installation aboard relay platforms.

ACIS system

The Advanced Combat Integrated SAR (ACIS) system, which Tadiran Spectralink introduced earlier this year, integrates the core elements of ASARS (the main location sensor, transceiver and datalink) with two new line-replaceable units. One of these, the AU- 700, incorporates a 225-300MHz UHF radio transceiver, direction- finding module, transponder unit, controller and interfaces. The other, the Advanced Integrated Mission Computer (AIMC), displays digital maps on its own screen and communicates with other aircraft- mounted systems. Its software, written mainly in the C++ object- oriented language running under Windows, can include situation- awareness and collision-avoidance modules.

The USAF awarded two CSAR-related contracts under the FY2000.1 solicitation of its Small Business Innovation Research program. GT Technologies is developing an innovative laser transponder that could be used by downed airmen to indicate their position at ranges out to 4km. The device is intended to be inexpensive, compact, lightweight and draw very little power. It combines high reflectance with polarization- independent operation, providing a broad- wavelength response and operating over a wide temperature range. The transponder is triggered by an aircraft-mounted laser, and can transmit voice and data.

Optical Energy Technologies is pursuing an alternative approach using an 810nm semiconductor diode laser and a silicon-array receiver/retroreflector, with their FOVs being mechanically scanned in a cross-track direction to the flightpath of the search aircraft over a width of 5,800ft (1.8km) or greater. The laser energy from the retro-reflector, which the survivor mounts on his head, provides a very large signal-to-noise ratio.

The system computer uses GPS-derived positional data to define the co-ordinates of the detected 'hit' and direct the pilot of the search aircraft.